Polar Modulation (PM) is a means of processing data so that it may be efficiently and effectively transmitted (by, for example, a Polar Transmitter). PM has several advantages over other available techniques in terms of achievable efficiency. PM makes possible the application of an amplitude modulation data signal at the very last stage of the Polar Transmitter, making it possible to reduce the current drain quickly as the transmit power level is reduced. In the context of handsets, for example, this has clear talk-time benefits.
In a Polar Transmitter, the data to be transmitted is separated into amplitude (a) and phase (p) signals. After separation, the phase signal (p) is applied to a phase modulator, and the amplitude signal (a) is applied to an Amplitude Modulator (AM). A digital PM, as opposed to an analog PM, has the advantage of a handling a high degree of digital content.
One example of a digital Amplitude Modulator (AM) which is utilized in a Polar Modulation scheme is a Radio Frequency Digital to Analog Converter (RFDAC). As described below, an RFDAC may be used to modulate an input in-phase/quad-phase (IQ) base band signal. Before the IQ base band signal is applied to the RFDAC, it is first divided into phase (ap) and amplitude (am) components. The amplitude component (am) is subsequently quantized, and applied to the RFDAC, whose RF input is separately modulated by the phase component (ap). However, the RFDAC has certain output receive band noise requirements. Quantization noise from the amplitude component (a) is a potential source of noise which must be addressed.
FIG. 1 shows a polar transmitter 100 including an RFDAC circuit 110, and digital signal processor circuit 120. The RFDAC circuit 110 is controlled by a digital amplitude signal (am), and driven by a phase modulated RF carrier signal (ap) generated by the digital signal processor circuit 120. Particularly, an input IQ base band signal (a) is first applied to a digital signal processor 10 which converts the analog IQ base band signal to digital (through Analog to Digital Converter (ADC) 11), and also transforms the signal into amplitude (am) and phase (ap) components (through Rectangular to Polar Converter (RPC) 12). In particular, the ADC 11 digitizes the input analog signal (a), and the RPC 12 translates the digitized wave into polar coordinates. RPC 12 outputs a digitized wave in polar coordinates, which takes the form R, P(sin) and P (cos), for example. In this example, the R coordinate represents an amplitude characteristic (am) of the digitized input wave. The P(sin) and P(cos) coordinates represent a phase characteristic (ap) of the digitized input wave.
The amplitude (am) and phase (ap) characteristics are then transmitted through separate paths in the RFDAC circuit 110. The amplitude characteristic (am) of the digitized input wave is modulated, via modulator 13, into digital pulses comprising a digital word (DW) quantized into, for example, bits B0 to BN, with a Most Significant Bit (“MSB”) to Least Significant Bit (“LSB”). The DW may be of varying lengths in various embodiments. In general, the longer the DW the greater the accuracy of reproduction of the input analog wave (a).
In the exemplary embodiment shown in FIG. 1, the digital amplitude signal (am) is converted into a N-bit (e.g., 7-bit) digital word by signal processor 13. Each bit of the N-bit digital word corresponds to a separate component control line am1-N (e.g., am1-7) at the output of the signal processor 13. Each of the component control lines am1-N are coupled to a separate control component 22 (e.g., switching transistors 22a-g) which feeds into another transistor 25 (e.g., 25a-g), which is turned ON or OFF depending on the particular bit value on the control component line. For example, if the DW corresponding to the digital amplitude signal (am) is “1110000”, the first three (3) transistors (e.g., 25a-c) will be biased ON, and the last four (4) transistors (e.g., 25d-g) will be biased OFF. In this manner, the amplification of the input analog signal (a) may be effectively controlled, as explained below.
The digital phase signal (ap) is modulated onto a wave by way of Digital to Analog Converter (DAC) 18 and synthesizer 20. The synthesizer 20 preferably comprises a Voltage-Controlled Oscillator (VCO) in the exemplary embodiment. The synthesizer 20 provides an output wave, which includes the phase information from the input wave (a). This output wave has a constant envelope (i.e., it has no amplitude variations, yet it has phase characteristics of the original input wave). The output wave may be further amplified by amplifier 24 before being provided to the plurality of transistors 25a-g on respective phase signal lines ap1-7.
Regulation of the transistors 25a-g may be accomplished by providing the digital word (DW) to the control components (e.g., switching transistors 22a-g). Each of the control components 22a-g preferably comprises a transistor acting as a current source. The control components 22a-g are switched by bits of the DW generated from the digital amplitude signal (am). For example, if a bit (e.g., the bit on line am1) of the DW is a logic “1” (e.g., HIGH), the corresponding control component (e.g., 22a) is switched ON, and so current flows from that control component to respective transistor segment (e.g., 25a). Similarly, if the same bit (e.g., the bit on line am1) of the DW is a logic “0” (e.g., LOW), the corresponding control component (e.g., 22a) is switched OFF, and so current is prevented from flowing through that control component to respective transistor segment (e.g., 25a). The current from all transistor segments 25a-g is then combined at the respective transistor outputs 26a-g, and provided as an output signal (b) on output signal line 27. Thus, by controlling the value of the DW, the amplification of the digital phase signal (ap) may be accurately controlled using the digital amplitude signal (am), thereby allowing reproduction of an amplified version of the input analog signal (a) at the output of the RFDAC circuit 110.
The conventional approach to improving receive band noise performance in the RFDAC is to introduce a radiofrequency (RF) filter, with suitable rejection in the receive frequency band, at the polar transmitter output (i.e., at a position downstream from the RFDAC). Inevitably, such a filter will have significant insertion loss in the transmit band, and hence, in order to maintain the desired overall transmit power level at the antenna, the power delivered from the power amplifier (e.g., RFDAC) to the RF filter must be increased accordingly. This increase in transmit power level demands an increase in current drain and hence the overall efficiency degrades.
Thus, there is presently a need for a polar transmitter (including an RFDAC) which has good receive band noise performance along with increased efficiency.